Aluminium is produced conventionally by the Hall-Heroult process, by the electrolysis of alumina dissolved in cryolite-based molten electrolytes at temperatures up to around 950.degree. C. In Hall-Heroult cells, the anodes are usually prebaked carbon blocks that are consumed by the electrochemical reaction, corroded by contact with the electrolyte and disintegrated by the evolved oxidising gases.
Prebaked anodes for aluminium production are made of a matrix of petroleum coke with pitch as binder. Their production involves various phases including preparing and treating the starting materials, mixing, forming and calcining at high temperature, followed by securing the current supply member by rodding.
The production of aluminium involves a complex reaction summarized by the relationship: EQU Al.sub.2 O.sub.3 +C.fwdarw.Al+CO.sub.2
with a theoretical consumption of 0.334 kg of the anodic carbon per kilo of product aluminium. However, the real anode consumption is 40-50% greater, and amounts to about 20% of the production cost of the aluminium.
The above-stoichiometric consumption of carbon stems from a series of secondary reactions or parasitic phenomena, subdivided as follows:
oxidizing reactions with oxygen from the air which contacts the upper part of the anode and, if the latter is non-protected, reacts (C+O.sub.2 .fwdarw.CO.sub.2); PA1 carbo-oxidation reactions with CO.sub.2 at the surface of the anode immersed in the electrolyte: the so-called "Boudouard equilibrium" (C+CO.sub.2 .fwdarw.CO); and PA1 selective oxidation of pitch coke with respect to petroleum coke, with consequent release of carbon particles which tend to deposit on the surface of the electrolyte, interfering with the electrolysis and increasing the electrolyte temperature.
In view of the importance of the anode consumption on the economics of the Al-production process, great efforts have been made in recent years to study the problem. This has led to the anode consumption being correlated with a series of variables, including the electrolyte temperature, the permeability of the anode to air, and the thermal conductivity of the anode. It is now possible, based on equations, to make estimates of anode consumption which correspond approximately to the values found in industrial practice.
It is widely believed that the major component of the increased anode consumption is due to oxidation of the anode surface in contact with air. A typical distribution of the specific net consumption of carbon (with the best state-of-the-art protective aluminium coatings) is:
______________________________________ Consumption kgC/kg Al % ______________________________________ Electrochemical -0.334 -76.0 Current efficiency -0.037 -8.4 Oxidation -0.051 -11.6 Carbo-oxidation -0.018 -4.0 Specific Net Consumption -0.440 ______________________________________
Prebaked carbon anodes contain metallic impurities originating from the starting materials, which impurities undesirably influence the anode consumption. In particular, V, Fe, S and especially Na exert a catalytic activity influencing the anode oxidation reaction, favoring the attack by O.sub.2.
Many attempts have been made to develop techniques to reduce the oxidation of prebaked carbon anodes in order to improve the efficiency, for instance by including additives in the coke-pitch mixture.
The addition of phosphorous, as phosphate or phosphoric acid, has a beneficial effect on anode consumption but undesirably pollutes the product aluminium and reduces current efficiency. For this reason, phosphorous-based treating agents such as that described in U.S. Pat. No. 4,439,491 have not been successful as oxygen inhibitors for prebaked carbon anodes used for aluminium production.
AlF.sub.3 has been proposed as additive on account of the fact that it is non-polluting to the bath. A reduction of the carbon consumption is obtained, but is attributed to the fact that AlF.sub.3 vapours reduce the differential reaction between coke and pitch, so the available saving is small because there is no reduction of the main oxidation.
Other compounds such as AlCl.sub.3 in an amount of 1-3%, or SiO.sub.2 as H.sub.2 SiO.sub.3 in an amount of 0.2 to 1%, have also been tried, but without giving satisfactory results.
Boron, principally in the form of B.sub.2 O.sub.3 and boric acid (H.sub.3 BO.sub.3) has also been found to inhibit the catalytic agents present (such as NaO.sub.2, FeO, and V.sub.2 O.sub.5) by forming stable alloys therewith.
By including boron compounds in prebaked carbon anodes with a concentration which in some cases is 0.2 to 0.3 weight % or more of the entire anode, it has been possible to reduce the oxidation by up to about 50%. See for example U.S. Pat. No. 4,613,375, which proposed adding 0.5 to 1.5 weight % of inorganic additives including B and B.sub.2 O.sub.3, and DE-A-35 38 294, which proposed doping the carbon making up the anode with manganese and boron or with cobalt and boron as corrosion inhibitors, with each element present in an amount of at least 0.1 and preferably at least 0.5 weight % of the carbon. These proposals, however, are still unsatisfactory because the maximum permissible boron content for an aluminium production anode should not exceed about 60 ppm in the product aluminium, which corresponds to a maximum of about 150 ppm in the entire anode.
For other applications, such as carbon anodes for arc furnaces, where boron pollution is not a problem, it has been proposed to improve the oxidation resistance by including boron in an amount of as much as about 3 weight % of the entire carbon body. See for example U.S. Pat. No. 4,770,825. Obviously, this is totally unacceptable for aluminium production anodes.
Protective coatings for aluminium production anodes have also been proposed, notably a fused layer of aluminium on the anode surfaces. This technique is economically questionable, as it requires from about 0.8 to 1.0 g Al/cm.sup.2 of the anode surface, and the poor wettability of carbon by the fused aluminium leads to problems in the uniformity of such coatings. Nevertheless, coating with aluminium has been the most widely accepted expedient to date to reduce anode oxidation.
Another proposed protective coating consists of alumina, but this has the disadvantage of creating a thermal insulation around the anode, leading to local overheating and acceleration of the oxidation process. protective coatings applied onto the carbon surfaces have
Attempts to coat the anodes with B.sub.2 O.sub.3 -based not been successful. U.S. Pat. No. 3,852,107 describes spraying a coating 0.5 to 5 mm thick onto a pre-heated anode; the spray mixture comprising a matrix of a boron compound and a refractory filler such as a carbide.
To overcome the drawbacks of previous attempts to make use of boric acid or salts thereof, DE-A-28 09 295 described coating a carbon body such as a prebaked anode for aluminium-production, by using a solution of ammonium pentaborate or ammonium tetraborate to produce a glassy coating of anhydrous boric acid (B.sub.2 O.sub.3). Such coatings initially reduce the reactivity of the anode surface with oxygen, but the effect is short-lived and, once the coating has been worn away, is lost.
Such coatings remain on an external surface of the anode and can easily be mechanically damaged during transport of the anode and its installation in the cell. Also, such coatings are not perfectly impervious to gas, and cannot protect the anode from oxidation.
Problems like those described above for prebaked carbon anodes apply also to the carbon cell sidewalls including a lower part submerged in the electrolyte and an upper part which is exposed to CO.sub.2 -enriched air, and which disintegrate and wear away as a result of attack by oxidising gases.